Polyhydric alcohols or polyols, such as trimethylolpropane or neopentyl glycol, have considerable economic significance as condensation components for formation of polyesters or polyurethanes, synthetic resin varnishes, lubricants and plasticizers. For this reason, various industrial methods which are supposed to lead to high-quality products at minimum expense have been developed. The emphasis of the developments lies essentially in the field of those process parameters that have a direct influence on the yield of desired main product. By contrast, there is no overall assessment of the polyol yield with simultaneous consideration of the efficiency of the overall reactant and energy input.
One possible preparation route for obtaining polyols includes the reaction of aldehydes with formaldehyde in aqueous solution. By the aldol addition mechanism, the aldehyde can at first form a methylol derivative of the corresponding aldehyde with formaldehyde in a first step. This addition reaction can be effected, for example, in the presence of catalytic amounts of bases or acids. Subsequently, in a second step, the aldehyde group can be reduced to the alcohol group with excess formaldehyde and with stoichiometric amounts of a base in a Cannizzaro reaction. In that case, a by-product formed at the same time is a stoichiometric amount of the formate of the added base. The corresponding formate salts are obtained as by-products and can be used, for example, as deicing agents, drilling aids or as auxiliaries in the leather industry.
For this basic reaction type, there are various different approaches to a solution in the patent literature relating to the preparation of polyols.
For example, the preparation of trimethylolpropane by an inorganic Cannizzaro process is disclosed in DE 1 182 646 A, WO 99/20586 A1, EP 2 341 041 A1, EP 1 323 698 A2, WO 2015/020796 A1 or WO 2015/020794 A1. The underlying reaction between n-butanal and formaldehyde is highly exothermic, and the heat released leads to disadvantageous temperature peaks which can affect the selectivity of the reaction and lead to colour problems in the end product. The reaction is typically conducted in the presence of a large amount of water with a correspondingly high heat capacity, in order to absorb the heat of reaction. A large amount of water is achieved through the use of a comparatively dilute aqueous formaldehyde solution, and through the use of aqueous solutions of inorganic bases.
According to DE 1 182 646 A1, it is likewise advantageous to use a high excess of formaldehyde in order to increase the yield of trimethylolpropane and to suppress the formation of unwanted by-products which can lead to colour impairments. Based on the n-butanal input, it is stated that it is possible to work with 6 to 10 moles of formaldehyde. It is also recommended that the inorganic base be used in an amount exceeding the amount theoretically required.
A mode of preparation with gradual addition of the reactants is disclosed, for example, in WO 2015/020796 A1. For preparation of trimethylolpropane, a reaction control in a tubular reactor is proposed, in which further co-reactants are added stepwise along the tubular reactor as the reaction progresses. At the same time, the tubular reactor is supplied with a formaldehyde-containing stream, and n-butanal and an aqueous solution of an inorganic base are added at various points in the tube over the length of the tubular reactor. The stepwise addition of n-butanal to the formaldehyde-containing stream constantly ensures a high excess of formaldehyde, based on n-butanal, and promotes selectivity in the trimethylolpropane direction. At the points where n-butanal and inorganic base are added, the tubular reactor may have static mixers or internals, in order to intensify the mixing after entry of the co-reactants and to remove the heat of reaction.
The known processes for preparing polyols by the Cannizzaro process in the presence of inorganic bases work with a considerable excess of formaldehyde, which is usually supplied to the reaction in the form of a dilute aqueous solution. After the reaction, therefore, a considerable amount of unreacted formaldehyde has to be removed from the crude product mixture. As a result of the high proportion of water, formaldehyde is removed as a highly dilute aqueous solution, for which a correspondingly high expenditure of energy is required.